Of course there are cheaper cloud services like B2 at $5/TB/mo, but even that can’t compete on price with physical infra when you have more than a handful of drives.
$15/TB is hard enough for a home buyer to reach for hard drives, excluding server cost. Where did you get $10?
$9.70/TB 10TB: https://www.ebay.com/itm/275400447467
$8.31/TB 4x8TB: https://www.ebay.com/itm/125132232253
You can get even cheaper if you buy in bulk.
Server cost is $1/TB with reasonable density
It’s econ101, the cost of providing the service doesn’t really factor into the price charged, what does factor in is the cost of alternatives (storage from azure, backblaze, on prem, not doing it), and the risks (increase s3 costs and people might realise they are being ripped off on ec2)